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Overview The cell membrane forms a barrier around the cell and
separates it from the outside environment
What is the main function of the cell membrane?
Allows certain materials and of the cell
The cell membrane is selectively permeable
Some substances cross the membrane easily while others can’t cross at all
The cell membrane is made of phospholipids and proteins, some with carbohydrates attached
Phospholipids Fatty acid “tails”
Hydrophobic – move away from water (polar)
Phosphate group “head”
Hydrophilic - move towards water (nonpolar)
Form two layers called a lipid bilayer
Phosphatehead
Fatty Acidtails
Phospholipid Bilayerpolarhydrophilicheads
polarhydrophilicheads
nonpolarhydrophobictails
Membrane Proteins Peripheral Proteins
Attached to the interior and exterior surface of the membrane by weak bonds
Can serve as ID tags for the cell
Integral Proteins Embedded in the bilayer and
stick out of the interior and exterior surface
Provide a pathway through the membrane Channels and Pumps
(permeases)
How Proteins Stay in the Membrane Membrane proteins have
polar and nonpolar ends
In the membrane Nonpolar amino acids
anchor the protein in the membrane
Outside the membrane Polar amino acids are
attracted to the fluid inside and outside the membrane
Marker Proteins Allows cells to recognize
each other
Important for the development of tissue and organs
Helps the immune system by recognizing cells that do not belong (bacteria) and attacking them
Have carbohydrate chains attached which serve as the markers
Passive Transport
Overview The cell needs to let some substances in and out in
order to maintain homeostasis Homeostasis is the stable internal conditions of a
living thing
Passive transport is the movement of substances across a membrane that requires no energy from the cell to occur
3 Types of Passive Transport Diffusion
Osmosis
Facilitated Diffusion
Diffusion Diffusion is the movement of
substances from HIGH to Low
concentration Diffusion happens along the
concentration gradient, which the difference in concentration across a space Molecules move down the
concentration gradient
Molecules diffuse until they reach equilibrium, where the concentration is equal on both sides Molecules will continue to diffuse,
but they will move back and forth equally
Not all molecules can diffuse Large molecules are too big to diffuse
Starches, proteins
Ions – atoms with a positive or negative charge
Salt, ammonia
Polar molecules – molecules with positive and negative ends
waterinside cell
outside cell
NH3salt
lipid
sugar aa H2O
Osmosis Water is special because
it is a polar molecule that can diffuse across the cell membrane because it is a solvent
Osmosis is the diffusion of water across a membrane Moves from high
concentration of water to low concentration of water
Direction of Water Movement The direction of water movement depends on solute
concentrations
Isotonic – equal solute, equal water
Hypertonic – more solute, less water
Hypotonic – less solute, more water
hypotonic hypertonic
water
Effect of Osmosis on Cells If a cell is in an isotonic
environment, it stays the same
If a cell is in a hypertonicenvironment, water diffuses out of the cell and the cell
If a cell is in a hypotonic environment, water diffuses into the cell and the cell and can burst
How Cells Deal with Osmosis Single-celled organisms, like paramecium, that live in freshwater
live in a hypotonic environment These cells use a contractile vacuole to pump excess water out of
the cell This requires energy
Plant cells usually live in a hypotonic environment Their cell wall’s are usually strong enough to resist the pressure
from the excess water inside the cell The pressure that water molecules exert against the cell wall is called
turgor pressure
In a hypertonic environment, water leaves a plant cell and the cell shrinks The cell membrane pulls away from the cell wall This is called plasmolysis and is the reason plants wilt when they
do not have enough water
Facilitated Diffusion Some molecules that cannot
diffuse across the membrane on their own are helped through the membrane
Special protein channels called carrier proteins help move molecules across the membrane Each protein is specific to the
molecule it moves No energy is required, so it is
still passive transport Molecules such as glucose and
other sugars, amino acids, salt, and ammonia are diffused by facilitated diffusion
high
low
Diffusion of Ions Ions can diffuse through the membrane by passing
through special proteins called ion channels
Some ion channels are always open
Others have “gates” that open or close
Gates will open or close in response to 3 kinds of stimuli
Stretching of the cell membrane
Electronic signals
Chemicals in the cytosol
Active Transport
Overview Sometimes cells need to move substances from low
concentration to high concentration
These substances move up the concentration gradient
Sometimes cells need substances that are too large to diffuse
To move these substances, cells must use energy
Active transport – the movement of materials across the cell membrane up the concentration gradient that requires energy by the cell
ATP
ATP
Sodium-Potassium Pump Sodium-Potassium pump is a carrier protein that
moves Na+ and K- ions up the concentration gradient
Pump helps maintain the concentration of Na+ higher outside the cell and the concentration of K- higher inside the cells
Cells need this in order to function properly
Endocytosis Endocytosis is the process
by which cells ingest fluid, macromolecules, and large particles
2 Types of Endocytosis
Phagocytosis
Pinocytosis
Phagocytosis Means “cell eating”
Moves large particles, such as food, from outside to inside the cell
Cell forms a “pouch” around the substance, which then pinches off and moves into the cell
Pinocytosis Means “cell drinking”
Fluid is taken into the cell in a process similar to phagocytosis
Exocytosis Exocytosis is the process where the cell releases large
substances, such as proteins
Substances are moved out of the cell
Vesicles move to the cell membrane, fuse with it, and release the contents